bims-cytox1 2017-11-19 papers

Issue of 2017‒11‒19
two papers selected by
Gavin McStay
New York Institute of Technology

Dis Model Mech. 2017 Nov 01. 10(11): 1343-1352

Early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in the KIKO mouse model of Friedreich ataxia.

Hong Lin, Jordi Magrane, Amy Rattelle, Anna Stepanova, Alexander Galkin, Elisia M Clark, Yi Na Dong, Sarah M Halawani, David R Lynch.

Friedreich ataxia (FRDA), the most common recessive inherited ataxia, results from deficiency of frataxin, a small mitochondrial protein crucial for iron-sulphur cluster formation and ATP production. Frataxin deficiency is associated with mitochondrial dysfunction in FRDA patients and animal models; however, early mitochondrial pathology in FRDA cerebellum remains elusive. Using frataxin knock-in/knockout (KIKO) mice and KIKO mice carrying the mitoDendra transgene, we show early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in this FRDA model. At asymptomatic stages, the levels of PGC-1α (PPARGC1A), the mitochondrial biogenesis master regulator, are significantly decreased in cerebellar homogenates of KIKO mice compared with age-matched controls. Similarly, the levels of the PGC-1α downstream effectors, NRF1 and Tfam, are significantly decreased, suggesting early impaired cerebellar mitochondrial biogenesis pathways. Early mitochondrial deficiency is further supported by significant reduction of the mitochondrial markers GRP75 (HSPA9) and mitofusin-1 in the cerebellar cortex. Moreover, the numbers of Dendra-labeled mitochondria are significantly decreased in cerebellar cortex, confirming asymptomatic cerebellar mitochondrial biogenesis deficits. Functionally, complex I and II enzyme activities are significantly reduced in isolated mitochondria and tissue homogenates from asymptomatic KIKO cerebella. Structurally, levels of the complex I core subunit NUDFB8 and complex II subunits SDHA and SDHB are significantly lower than those in age-matched controls. These results demonstrate complex I and II deficiency in KIKO cerebellum, consistent with defects identified in FRDA patient tissues. Thus, our findings identify early cerebellar mitochondrial biogenesis deficits as a potential mediator of cerebellar dysfunction and ataxia, thereby providing a potential therapeutic target for early intervention of FRDA.

Keywords: Cerebellum; Friedreich ataxia; Mitochondrial biogenesis; Neurodegenerative diseases; Respiratory chain complex

DOI: https://doi.org/10.1242/dmm.030502

Genome Biol Evol. 2017 Nov 08.

Genetic drift and indel mutation in the evolution of yeast mitochondrial genome size.

Shujie Xiao, Duong T Nguyen, Baojun Wu, Weilong Hao.

Mitochondrial genomes (mitogenomes) are remarkably diverse in genome size and organization, but the origins of dynamic mitogenome architectures are still poorly understood. For instance, the mutational burden hypothesis postulates that the drastic difference between large plant mitogenomes and streamlined animal mitogenomes can be driven by their different mutation rates. However, inconsistent trends between mitogenome sizes and mutation rates have been documented in several lineages. These conflicting results highlight the need of systematic and sophisticated investigations on the evolution and diversity of mitogenome architecture. This study took advantage of the strikingly variable mitogenome size among different yeast species and also among intraspecific strains, examined sequence dynamics of introns, GC-clusters, tandem repeats, mononucleotide repeats (homopolymers) and evaluated their contributions to genome size variation. The contributions of these sequence features to mitogenomic variation are dependent on the timescale, over which extant genomes evolved from their last common ancestor, perhaps due to a combination of different turnover rates of mobile sequences, variable insertion spaces and functional constraints. We observed a positive correlation between mitogenome size and the level of genetic drift, suggesting that mitogenome expansion in yeast is likely driven by multiple types of sequence insertions in a primarily non-adaptive manner. Although these cannot be explained directly by the mutational burden hypothesis, our results support an important role of genetic drift in the evolution of yeast mitogenomes.

Keywords: GC-cluster; Genome size; genetic drift; intron; mitogenome; tandem repeats

DOI: https://doi.org/10.1093/gbe/evx232